Mike Taylor recently sent me an email asking for a larger version of a figure I once published in a book chapter. Naturally, I promised to send it to him. But, being away from my computer, with email available only via my phone, I couldn’t send it right away. And promptly forgot.

Thus, Mike had to go ahead and publish the planned blog post without my figure. Here’s the link.

Now, I finally remembered, and through sheer persistence managed to find my original submitted files in my sorry heap of data backup. Here goes – starting with a small part of a big figure, but a part that shows nearly all that needs to be shown to make Mike’s point:

This is a CAD model of Diplodocus I built long ago. It is rough, and takes air spaces within the body into account in a fairly rough way, but – and that’s a big issue with dinosaur models – there is enough soft tissue mass modelled onto the tail, especially its base.

I used a rather complex multibody dynamics program to do all kinds of shit with the model, but first of all I had it calculate the position of the center of mass (COM). I won’t bore you with the full story; suffice to say that I put a small white dot in the figure to show the result.

Yes, the COM is that close to the hind limbs.

Now, one model alone would be bad practice, so I ran a bunch of variations. As long as my assumptions about density and volume stayed reasonable, the COM stayed really close to the hind limbs.

The next thing I did is look at another sauropod, one with a quite different overall look: Giraffatitan. Unsurprisingly, a much shorter and thinner tail and much longer and stouter forelimbs meant the COM came to rest somewhere else entirely. You’ll see in a moment…..

Then, I went ahead with the work I had set out to do originally: look at the ability of the two sauropods to rear into an upright stance. Not just rear up and come back down with a thump! right away thanks to gravity, but the ability to adopt a bipedal pose that puts the head high up for feeding on large trees. A position that must thus be held for quite a while, which in turn requires that it is inherently stable, that you can get there easily, and that you can get back down speedily, too.

Why inherently stable? Well, think of a ladder and how you behave on it. When you are high up on a ladder you will either be careful and restrict your movements, or you will fall. If you’re positioned so that the tiniest motion unbalances you, there is no way you can do useful work for a prolonged time. For a sauropod poking its head into a mass of branches, grabbing them and pulling vigorously most certainly was an activity that required it to be posed so that it did not constantly worry about toppling over.

Why easy to reach? Because if it is really hard to reach, harder to reach than a similar pose is for an elephant, then the effort to get there makes it so difficult to use that it is no good for regular behaviour. Sure, you can do the weirdest stuff, like walking on your hands. But every day?

And why is getting back down so important? That is in fact the simplest point: if a big theropod ambles by, you do not want to spent several minutes carefully letting yourself back down into a pose in which you can deal with the threat.

So, I modelled on, and here are the figures as the appear in full in the article:

This is one way Diplodocus can easily get into an upright position. By pushing its butt backwards a bit, flexing the knees a tiny bit, the COM comes to rest right above the hind feet. Now, all it takes is a very slight rotation in the hip joints, easily achieved by the strong caudofemoral muscles of both sides acting together, and the entire animal minus the legs starts rotating. When the tail hits the ground (slowly and softly) it is time to stop. There you go – feeding height doubled, or tripled, depending on how mobile you believe the neck is in extension.

Back down is also easy: just a push forward with the forelimbs to get some momentum, and slowly relaxing the caudofemoralis muscles and the animal is back down in a few seconds, ready to fight or run away (well, amble away that is).

In between, all is well balanced, because the limbs that can counter a slight imbalance easily, the hind limbs, are attached to the body at roughly the height the center of mass is at. Motions of the COM can therefore be countered quickly, before the shit hits the fan. If there was a long lever arm between the point where the animal can influence the trunk’s position relative to the limbs and the COM, it would be a very difficult task to fiddle things into equilibrium. Additionally, in lateral view the angle between the line connecting the edge of the support area and the COM and the vertical through the COM is larger the further down the COM is, meaning higher stability as well.

Giraffatitan left, Diplodocus right. Guess who’s standing stably, and who’s more like a drunk on a ladder?

Various poses I tried for Giraffatitan. The COM is shown by a tiny sphere in the middle of the body. High, high up, far away from the hip joints. This doesn’t look good for balance. Add to that the emaciated tail base and thus weak caudofemoralis muscles of Giraffatitan, and you get an animal that was really bad at rearing up and staying there. Diplodocus, on the other hand, was (I concluded) well capable of rearing up and staying there for an extended feast.

So, what does that mean for Mike’s bipedal Diplodocus? For one thing, the COM is in the right place. The other thing is that a bipedal pose would have required somewhat flexed hind limbs. Not something I guess the animal did for a long time. And there are lots of issues with walking bipedally, not least the issue of retaining steering. Obviously, one can use various tricks for keeping the body position stable, but why bother if there is an easier way? Even a paltry 10% of body weight supported by a forelimb placed well forward of the hind feet would help a tremendous lot with going in the actual direction you want to go. So bipedal Diplodocus yes, but not regularly!

11 Responses to Addendum to SV-POW!’s “SO close”

A question that occurred to me many years ago which you may be able to shed some light on – I noticed that the chevrons on the underside of Diplodocus‘ tail commence at a point before where the tail is able to contact the ground (even in Hatcher’s droopy-tail reconstruction of D. carnegiei). However, that part of the tail could be brought into contact with the ground if Diplodocus were to rear up on its hind legs.

It makes me wonder whether the chevrons reinforced the tail so that it could act a bit like the third leg of a tripod whilst protecting blood vessels and nerves. Is that wild speculation or do you think that there might be something to that line of thought?

Mark,
to my surprise it is kinda hard to to get that area of the tail onto the ground when the hind limbs are fairly erect. So either my COM calculation is way off, or rearing for feeding was done with minimal tail support.
Other activities, like mating, may have seen significant tail support though, and that’s where those odd chevrons likely come in 🙂

Hey, thanks for that. I think that your COM calcs are unlikely to be too far out but I guess that the tail could still be used to assist with maintaining balance even if not actually providing much on the way of firm support.

Could call re mating. We can be fairly sure that they managed it but “How could…I mean… I don’t even…” 😉

Hmm, fantastic and interesting post Heinrich. What about Mamenchisaurus? Didn’t have an upward kink in its tail? How would that affect its rearing capabilities? Would it end up being possible for Mamenchi to pose as in John Conway’s reconstruction showed it (posted somewhere on SVPOW), or would such weight and the already great neck length make it much more costly to rear?